We present results on J/psi production in muon interactions with tin and carbon targets at incident muon energies of 200 and 280 GeV. The ratio of cross sections per nucleon for J/psi production on tin and carbon, R(Sn/C), is studied as a function of p(T)2, z and x. We find an enhancement for coherent J/psi production R(coh)(Sn/C) = 1.54 +/- 0.07, a suppression for quasielastic production R(qe)(Sn/C) = 0.79 +/- 0.06 and for inelastic production R(in)(Sn/C) = 1.13 +/- 0.08. The inelastic cross section ratio can be interpreted within the Colour Singlet model as an enhancement ot the gluon distribution in tin with respect to that in carbon. The dependence of the ratio on z and p(T)2 can explain the discrepancy between the results obtained in previous experiments.